I'd like to see this feature too.
No matter what you're doing it's always easy to take a few photos aligned to some reference plane that is useful to you, and then use them for alignment.
Quite often you might just drop the camera onto a tripod for example and take a 'flat' shot purely for quick alignment purposes right inside Photoscan, rather than faffing in Cloud Compare or whatever else.
Trying to align by eye with the mouse in Photoscan on the other hand, is quite a slow/inaccurate process.
Dave